The binary iron-cobalt alloys with a cobalt content of between 33 and 55% by weight are extraordinarily brittle, which is attributable to the formation of an ordered superstructure at temperatures below 730° C. The addition of approximately 2% by weight of vanadium impedes the transition to this superstructure, so that relatively good cold workability can be achieved after quenching to room temperature from temperatures of over 730° C.
Accordingly, a known ternary base alloy is an iron-cobalt-vanadium alloy which contains 49% by weight of iron, 49% by weight of cobalt and 2% by weight of vanadium. This alloy has long been known and is described extensively, for example, in “R. M. Bozorth, Ferromagnetism, van Nostrand, New York (1951)”. This vanadium-containing iron-cobalt alloy is distinguished by its very high saturation induction of approx. 2.4 T.
A further development of this ternary vanadium-containing cobalt-iron base alloy is known from U.S. Pat. No. 3,634,072, which describes, during the production of alloy strips, quenching of the hot-rolled alloy strip from a temperature above the phase transition temperature of 730° C. This process is required in order to make the alloy sufficiently ductile for the subsequent cold rolling. The quenching suppresses the ordering. In manufacturing terms, however, the quenching is highly critical, since what are known as the cold-rolling passes can very easily cause fractures in the strips. Therefore, considerable efforts have been made to increase the ductility of the alloy strips and thereby to increase manufacturing reliability.
Therefore, U.S. Pat. No. 3,634,072 proposes, as ductility-increasing additives, the addition of 0.02 to 0.5% by weight of niobium and/or 0.07 to 0.3% by weight of zirconium.
Niobium, which incidentally may also be replaced by the homologous element tantalum, in the iron-cobalt alloying system, not only has the property of greatly reducing the degree of order, as has been described, for example, by R. V. Major and C. M. Orrock in “High saturation ternary cobalt-iron based alloys”, IEEE Trans. Magn. 24 (1988), 1856-1858, but also inhibits grain growth.
The addition of zirconium in the quantity of at most 0.3% by weight proposed by U.S. Pat. No. 3,634,072 likewise inhibits grain growth. Both mechanisms significantly improve the ductility of the alloy after quenching.
In addition to this high-strength niobium- and zirconium-containing iron-cobalt-vanadium alloy which is known from U.S. Pat. No. 3,634,072, zirconium-free alloys are also known, from U.S. Pat. No. 5,501,747.
That document proposes iron-cobalt-vanadium alloys which are used in fast aircraft generators and magnetic bearings. U.S. Pat. No. 5,501,747 is based on the teaching of U.S. Pat. No. 3,364,072 and restricts the niobium content disclosed therein to 0.15-0.5% by weight. Furthermore, U.S. Pat. No. 5,501,747 recommends a special magnetic final anneal, in which the alloy can be heat-treated for no more than approximately four hours, preferably no more than two hours, at a temperature of no greater than 740° C., in order to produce an object which has a yield strength of at least approximately 620 MPa. This is very limiting and also very unusual, since the soft-magnetic iron-cobalt-vanadium alloys are normally annealed at temperatures of over 740° C. and below 900° C.
The magnetic and mechanical properties can be adjusted by means of the annealing temperature. Both properties are crucial for use of the alloys. However, it is very difficult to simultaneously optimize these two properties, since the properties are contradictory:
1. If the alloy is annealed at a relatively high temperature, the result is a coarser grain and therefore good soft-magnetic properties. However, the mechanical properties obtained are generally relatively poor.
2. On the other hand, if the alloy is annealed at lower temperatures, better mechanical properties are obtained, on account of a finer grain, but the finer grain results in worse magnetic properties.
A major drawback of the alloy selection disclosed by U.S. Pat. No. 5,501,747 is the need for the abovementioned rapid anneal, which may only be carried out for approximately one to two hours at a temperature close to the ordered/unordered phase boundary in order to achieve usable magnetic and mechanical properties.
If there is a very large quantity of material to be annealed, reliable production can therefore only be realized with very great difficulty, on account of different heat-up times and on account of temperature fluctuations within the material to be annealed. On a large industrial scale, the result is generally unacceptable scatters with regard to the yield strengths which are characteristic of the mechanical properties.